1. Field of the Invention
This invention relates to a process for producing terephthalic acid. More specifically, it relates to a process for producing terephthalic acid providing terephthalic acid of high quality capable of being used to produce a polymer of good color when used in direct polymerization, and in which loss of the reaction solvent by combustion is reduced.
2. Description of the Prior Art
Various methods have been suggested heretofore for producing terephthalic acid of high quality for use in direct polymerization by simply oxidizing a para-dialkylbenzene and/or an oxidized intermediate thereof with molecular oxygen or a molecular oxygen-containing gas in the liquid phase in the presence of an oxidation catalyst in a lower aliphatic carboxylic acid solvent (e.g., as disclosed in Japanese Patent Publication No. 36732/70, U.S. Pat. No. 3,846,487 and Japanese Patent Application (OPI) Nos. 106833/77, 26240/74, 80531/73 (The term "OPI" as used herein refers to a "published unexamined Japanese patent application"), etc.). These prior methods, however, are not entirely satisfactory for the production of high quality terephthalic acid for direct polymerization. For example, a large loss of the reaction solvent occurs due to combustion. Or when the resulting terephthalic acid is used to produce a polyester by the so-called direct polymerization method which comprises esterifying the terephthalic acid directly with ethylene glycol without proceeding through dimethyl terephthalate, and polycondensing the esterification product, e.g., as disclosed in Japanese Patent Publication No. 19314/75, the resulting polyester does not always have uniform color.
In recent years, the direct polymerization method has become increasingly used for production of polyesters, and the trend is toward the employment of a continuous direct polymerization method which is economically more advantageous than the batchwise method. Under these circumstances, the conditions as to quality required of the starting terephthalic acid are more rigorous, and the quality of the starting terephthalic acid should be so high and uniform that its use in the direct polymerization method provides over long periods of time polyesters having an industrially reproducible uniform quality and good color. However, since the oxidation reaction of a para-dialkylbenzene and/or an oxidized intermediate thereof for the production of terephthalic acid is a radical chain reaction, various impurities may be formed in addition to the reaction intermediates or colored materials and may be included in the final terephthalic acid. Such terephthalic acid when used results in polyesters with poor color when directly polymerized continuously. This inconvenience frequently occurs in the conventional method for producing terephthalic acid for direct polymerization depending upon the reaction conditions and is especially frequent when terephthalic acid is produced continuously over relatively long periods of time.
In order to obtain polyesters with uniform quality and good color by direct polymerization, it is important to control the production of terephthalic acid used as a starting material. Simply checking the 4-carboxybenzaldehyde content of the desired terephthalic acid, and determining the color as a solid or a solution are insufficient as is the case using conventional techniques. Terephthalic acid having a low 4-carboxybenzaldehyde content and good color as a solid or solution sometimes results in a polyester having poor color when such is produced by the direct polymerization method.